Generally speaking, a discharge lamp is formed by attaching an electrode and a base, which may be made of plastic, to each end of a glass bulb and fitting the bases to a socket that is mounted on the body of a lighting fixture. When such a discharge lamp is close to the end of its life, it causes a symmetric operating condition, which is an abnormal discharge that heats the regions around the electrodes. This causes a particularly serious problem in case of a discharge lamp having a small diameter glass bulb, which is becoming more commonplace. As the distance between each electrode and the glass bulb of a small diameter-type lamp is minimal, such an abnormal discharge tends to increase the temperature of the glass bulb excessively and often presents the danger of the glass bulb, the plastic bases or the socket melting.
An example of discharge lamp lighting devices of this type is shown in FIG. 26.
The discharge lamp lighting device 1 shown in FIG. 26 includes a commercial AC power source e, a full-wave rectifying circuit 2 having an AC input terminal connected to the commercial AC power source e, a DC/DC converter 3 connected to a DC output terminal of the full-wave rectifying circuit 2, an inverter circuit 4 that serves as a high frequency generating means and is connected to the DC/DC converter 3, and a load circuit 5 connected to the inverter circuit 4.
The load circuit 5 is connected via an inductor L1, which serves as a current limiting element, to a fluorescent lamp FL serving as a discharge lamp. A capacitor C1 is connected in parallel with the fluorescent lamp FL. Also connected in parallel with the fluorescent lamp FL is a life-end detecting circuit 6 that is connected to the inverter circuit 4 and controls the inverter circuit 4, thereby serving as a life-end detecting means. With the configuration as above, fullwave rectification of the AC voltage from the commercial AC power source e is conducted by the full-wave rectifying circuit 2, and the rectified voltage is then converted to DC voltage as it is smoothed and adjusted by the DC/DC converter 3. When this DC voltage is input to the inverter circuit 4, the inverter circuit 4 generates a high frequency voltage at a given frequency, which is applied to the load circuit 5. At the load circuit 5, the high frequency voltage input thereto is applied via the inductor L1 to the fluorescent lamp FL and the capacitor C1. The fluorescent lamp FL and the capacitor C1 generate an appropriate resonance so that a high voltage necessary for the starting up of the fluorescent lamp FL is applied to the fluorescent lamp FL, thereby lighting the fluorescent lamp FL.
Throughout the period when the fluorescent lamp FL is in the `on` state, the life-end detecting circuit 6 monitors the voltage between the electrodes of the fluorescent lamp FL. When the fluorescent lamp FL1 comes close to the end of its life, the life-end detecting circuit 6 detects the end of the life and controls the inverter circuit 4 to stop its function.
The load characteristics of the load circuit 5 of the discharge lamp lighting device 1 shown in FIG. 26 are shown in FIG. 27, wherein the curve A and the curve B respectively represent the load characteristic curves in the full-intensity illumination mode and in the dimming mode, while the curve c and the curve d respectively represent the operating characteristics of a fluorescent lamp FL functioning in normal conditions and a fluorescent lamp FL close to the end of its life.
The curves representing load characteristics of the load circuit 5 in the full-intensity illumination mode and in the dimming mode have arcs of a similar shape. As the fluorescent lamp FL approaches the end of its life, the lamp voltage gradually increases, and the operating characteristics of the load circuit 5 move upward in the chart of FIG. 27. When the fluorescent lamp FL is operating in normal conditions in the full-intensity illumination mode, the fluorescent lamp FL functions at the point X1, which is the point of intersection between the curve A and the curve c. During the dimming mode, the fluorescent lamp FL functions at the point X2, which is the point of intersection between the curve B and the curve c. In other words, the power output in the dimming mode is lower than in the full-intensity illumination mode.
When the fluorescent lamp FL reaches the end of its life during the full-intensity illumination mode, the fluorescent lamp FL functions at the point Y, which is the point of intersection between the curve A and the curve d, and remains lit in the half-wave discharge condition. This presents the danger of melting of parts or other problems.
As the inverter circuit 4 stops functioning, the fluorescent lamp FL becomes dark. This may also causes security problems.
Conventionally known discharge lamp lighting devices which may overcome the above problems include those that are adapted to light a plurality of fluorescent lamps in such a manner as to extinguish only the lamps that are not working properly while maintaining the normal ones lit. An example of such discharge lamp lighting devices is disclosed in Japanese Patent Laid-Open No. 231295/1989. The discharge lamp lighting device disclosed in said Japanese Patent Laid-Open No. 231295/1989 calls for connecting a plurality of fluorescent lamps in parallel with one another and, upon detecting one or more abnormal lamps when said plurality of fluorescent lamps are lit, reducing the output from the inverter circuit to such a level that the other lamps, i.e. those operating in normal conditions, can remain lit.
In other words, if there is any fluorescent lamp that has reached the end of its life, the discharge lamp lighting device keeps the other fluorescent lamps lit, which are still in normal working conditions, in the state where the output from the high frequency generating means is reduced through the reduction of the output from the inverter circuit. Thus, the minimum necessary illumination level is ensured.
However, should the discharge lamp lighting device disclosed in Japanese Patent Laid-Open No. 231295/1989 be applied to small diameter-tube-type fluorescent lamps, the temperature of the glass bulb of an abnormal lamp would be still too high in spite of the reduced output from the inverter circuit. Furthermore, reducing the output to such a level as to prevent the fluorescent lamp at the end of its life from continuing the discharge makes it difficult to keep the normal fluorescent lamps lit. It is particularly difficult to keep normal fluorescent lamps of a household lighting fixture lit, because a household lighting fixture is normally designed such that a single inverter circuit lights two or more fluorescent lamps which have different rated power consumption.